Floating grids for fluorescent lamps



NOV. 29, 1955 J. J. MASON FLOATING GRIDs FOR FLUoREscENT LAMPS Filed April 25J 1951 |NVENTOR JT J" Maso/V United States Fatemi FLATING GRIDS FR FLURESCENT LAMPS Julien J. Mason, WestCaldweil, N. l., assigner to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application April 25, 1951, Serial No. 222,317

2 Claims. (Ci. SiS-41M) This invention relates to grids and, more particularly, to electrically floating grids for fluorescent lamps and similar low pressure gaseous discharge tubes.

The anode voltage drop in a gaseous discharge tube results from a negative space charge surrounding the anode. This voltage drop is a measure of the energy that must be given to the electrons to overcome the repelling force of the space charge. It results in kinetic energy of the electrons which is dissipated at the anode in the form of heat. The density of the space charge and the magnitude of the anode voltage drop increase as the anode area is decreased. If, however, the space charge voltage becomes high enough, the kinetic energy of the electrons becomes sufficient to cause ionization just in front of the anode, thus temporarily relieving the negative space charge and reducing the voltage drop. As the newly formed positive ions are lost to the anode yregion both by recombination and by acceleration toward the cathode, the space charge reforms and the processr repeats. In conventional fluorescent lamps, this usually results in an oscillatory, voltage drop having an amplitude f oscillation of about l0 volts which is roughly the ionization potential of mercury.

The above-mentioned dissipation of electron kinetic energy at the relatively small anode of a conventional fluorescent lamp is not only wasteful, but may result in excessive anode temperature and vaporization of metal onto the envelope, thus causing end darkening and reduced lamp life. A small increase in anode area may actually increase the eicctive anode voltage drop by vreducing the space charge to a stable value, such that the relief provided by periodic ionization at the anode surface does. not occur.

While a very substantial increase in anode area may bel effective in reducing anode voltage drop, other practical diiculties arise. First, a large anode, unless placed in a less effective position well behind the conventional electrode, produces an objectionable shadow at the lamp end and may reduce light output by absorption. Secondly, two large anodes are required at each end of the lamp if polarization of the base pins is to be avoided. Finally, the filamentary electrodes, which serve as anodes on alternate half cycles, receive less heat on the anode half cycle for maintaining the emitting temperature of the electron emissive hot spot and the cathode drop is increased. Whileit is desirable from the standpoint of lamp eliciency to minimize the dissipation of energy at the anode, it is also desirable to concentrate unavoidable losses (such as the heat of electron condensation which depends on the work function of the anode surface) on the larnentary electrode rather than on an auxiliary anode surface where they serve no useful purpose.

Hence, it has been found advantageous, according to my invention, to reduce the anode voltage drop and increase the light output without increasing the anode area 0f a conventional low pressure gas. I accomplish this by providing in front of each tilamentary electrode an electrically floating grid which produces intensified ionization close to the anode surface and prevents formation of the usual space charge.

In its general aspect, the present invention has the object of overcoming the aforementioned disadvantages of the prior art uorescent lamps and similar low pressure gaseous discharge tubes. Specifically, an object of the present invention is to increase the eiiciency of uorescent lamps and similar low pressure gaseous dis.- charge tubes through a reduction of the anode voltage drop.

Another specific object is to improve the appearance of uorescent lamps and similar low pressure gaseous discharge tubes by increasing light output at the ends.

A further object is to reduce the possibility ofv overheating of the anode and resulting end darkening' and reduced life of fluorescent lamps and similar low pressure gaseous discharge tubes.

Other objects and advantages of the invention will appear to those skilled in the art to which it appertains as the description proceeds, both by direct recitation thereof and by implication from the context.

Referring to the accompanying drawing, in which like numerals of reference indicate similar parts throughout the separate views:

Fig. 1. is an elevational View of a fluorescent lamp with the iluorescent coating on the envelope broken away to show at each end the larnentary electrode and the electrically floating grid of my invention;

Fig. 2 is an elevational view of an electrode mount for the lamp showing anV embodiment of the electrically floating grid of my invention;

Fig. 3 is an elevational View similar to Fig. 2, but showing an alternative embodiment of an electrically floating grid of kmy invention;

Fig. 4 is a top view of the grid of Fig. 3;.

Fig. 5 isa typical graph of experimental results ohtained with a short discharge tube containing a movable grid and showing lamp voltage drop as ordinates and grid-anode spacing as abscssas.

In Fig. 1 the reference numeral 10' designates a tubular vitreous'envelope of a uorescent lamp l1. While a iluorescent lamp has been selected as an embodiment ofi the present invention, it will be understood that the invention is not restricted to use with iluorescent lamps. This envelope l@ is coated'on its inner surface with uorescent material l2 andv has a iilarnentary electrode mount 13- sealed into each end. Such mount comprises a stern 14., consisting of a flare 15, two electrode leading-in conductors 16, suitably three-piece leads, a dummy lead 7, suitably a two-piece lead, and a lamentary electrode l; coated with emissive material and mounted on said conductor 16 as shown in Fig. 2.

It' will also be understood that in a fluorescent lamp employed here as illustrative of a low pressure gaseous discharge tube of the positive column type, the filamentary electrode Li serves as cathode on one half cycle and as anode on the other half cycle of operation on an alternating current system.

The flare l5 is a piece of glass tubing having one end flared axially outwardly and having the sealing portions, suitably Dumet, of conductors 16 sealed through a press i9 at the other end. Dumet is the trade name for wire consisting of a nickel-iron core enclosed in a copper sheath having a total thickness approximately 25% of the finished wire diameter. The interior portions of conductors i6 may be suitably nickel and the exterior portions may be copper. Dummy lead i7 need not eX- tend through the press .t9 of stem 14. As in the case of leading-in conductors 1.6 the interior portions of the two-piece dummy lead 17 are appropriately nicke1 and the sealing portion Dumet.

One of the electrode mounts 13 has a suitable tubulation 20 for evacuating the envelope and the admission of argon or other suitable inert gas and mercury during the processing of said lamp 11. After the exhaust which may consist of preliminary evacuation, bake, electrode treating, mercury and inert gas ll, and tip-off, bases 21 are affixed to the lamp 11 at both ends of the envelope.

According to my invention, a flat grid perpendicular to the axis of the tube or a cage grid effectively surrounding the conventional electrode may be employed. To prevent the discharge from going around the grid, a at grid in the form of a disc 22, as shown in Fig. 2, has a diameter as close to that of the envelope 11 as is practical. This grid 22 is composed of metallic Wires, woven or otherwise held together to form a mesh which suitably may have square openings of 1/16 to 1A" on a side. Ideally, the grid wires should be as small as possible to hold to a minimum the surface area upon which electrons and positive ions can re-combine, but to l0 mil wires are satisfactory. While metallic grid wires have been used, electrical conductivity is not necessary and fibres of insulating material would serve the same purpose. Grid 22 is supported in fixed space relationship with electrode 18 by means of an Lshaped support wire 23, which is welded or otherwise joined both to grid 22 and dummy lead 17.

In Figs. 3 and 4 an electrically floating cage grid 24, an alternative embodiment of my invention, is shown surrounding the electrode 18. This grid is formed by mounting, as by welding, the ends of U-shaped wires 25, suitably 5 to 10 mil diameter, on a suitable metallic ring 26. Grid 24 is mounted on dummy lead 17 by means of a metallic connector wire 27, so that electrode 1S is centered within the hemispherical end portion 28 of grid 24, thereby providing uniform spacing between electrode 18 and grid 24.

An electrically floating grid, such as disc 22 or cage 24, in the positive column of a fluorescent lamp discharge receives a negative charge in the same manner as a probe. Therefore, it retards electrons approaching it from the cathode side and these electrons must possess a certain minimum energy in order to get through the grid. Low speed electrons accumulating on the cathode side of the grid retard oncoming fast electrons and therefore receive the necessary additional energy from them. As a result, the range of electron energies existing in the positive column is greatly constricted at the grid. On the anode or positive potential side of the grid the electrons are rapidly accelerated and, because of the uniformity of their velocities, they ionize more or less as a group a short distance beyond the grid. When the grid-anode spacing is such that this intensified ionization occurs just in front of the anode, the positive ions so produced relieve the negative space charge and thereby reduce the anode voltage drop.

The effect of such a grid in reducing the anode voltage drop is demonstrated by the curve of Fig. 5. This plot of lamp voltage drop against grid anode spacing in inches for a constant direct current was obtained with a short experimental lamp containing a movable flat grid. The voltage drop of the lamp Without a grid is approximately represented at A, where the grid is too far from the anode to have much effect. At D the grid is in its most advantageous position, thereby reducing the anode drop by about 5 volts. The anode space charge is practically eliminated at D, while it reaches a maximum stable value at C. Between A and C the space charge tends to exceed the value at C and is periodically relieved by ionization at the anode. Therefore, the portion AC of the curve of Fig. 5 represents effective values of voltage oscillating between nearly constant limits which are approximately represented by the voltages at C and D. There are no oscillations to the left of C.

As the grid is moved from B to D, continuously increasing relief of the negative space charge at the anode is provided by the continuously approaching positive space charge near the grid. The voltage drop increases along the BC portion of the curve because the small relief provided by the grid increases the stability of the anode space charge and interferes with the periodic but rather complete relief afforded by the anode voltage oscillations. The frequency of these oscillations decreases until they are completely eliminated at C. Then the further reduction of the anode space charge by the grid reduces the voltage drop along the CD portion of the curve. Where the grid is too close to the anode, however, ionization in front of the grid is reduced and the lamp voltage drop rises as shown between D and E on the curve.

The reason for the reduction in voltage between A and B becomes clear when the striating effect of the grid on the discharge is understood, since the electrons give up most of their energy in mass ionization close to the grid, they are not capable of ionizing again immediately. Therefore, when the grid is a considerable distance from the anode, the positive ion space charge near the grid is followed by a negative space charge on the anode side. with the grid at B, this negative space charge adds to that space charge normally at the anode, thereby increasing its instability and enhancing anode oscillations.

in the practical design of a low pressure gaseous discharge tube, of the positive column type such as the uorescent lamp 11, the grid 22 is permanently mounted in its most advantageous position with respect to electrode 18 of each mount 13. The most effective anode-grid spacing depends on the gas till employed and somewhat on the grid construction itself. For a fluorescent lamp containing argon in the pressure range of 2.0 to 3.6 mm. of mercury, this anode grid spacing generally falls between 0.2 in. and 0.3 in.

In addition to reducing the anode voltage drop such grids as have been described above also increase the light output. This is a direct result of the increased ionization provided on the anode side by both the grid at the cathode end of the lamp and the grid at the anode end. At the cathode end the effect consists of a reduction in the length of the Faraday dark space.

Although embodiments of my invention have been disclosed, it will be understood that modifications may be made within the spirit and scope of the appended claims.

l claim: Y

In a low pressure gaseous discharge device of the positive column type a iilamentary electrode coated with emissive material and an electrically floating and nonconducting flat grid having a mesh size in the range of 4 to 16 meshes per inch, said grid disposed in front of said electrode and having a spacing from said electrode in the range of 0.2 to 0.3 inch to reduce the discharge voltage by relief of the negative space charge adjacent said electrode and to increase the brightness adjacent said electrode and the efficiency of the discharge by enhanced ionization when said electrode is serving as the anode.

2. In a low pressure gaseous discharge device of the positive column type a filamentary electrode coated With emissive material, and an electrically iloating at grid disposed in front of said electrode and having a mesh size in the range of 4 to 16 meshes per inch and a spacing from said electrode in the range of 0.2 to 0.3 inch to reduce the discharge voltage by relief of the negative space charge adjacent said electrode and to increase the brightness adjacent said electrode and the eiciency of the discharge by enhanced ionization when said electrode is serving as the anode.

References Cited in the file of this patent UNITED STATES PATENTS 1,980,534 Kirsten Nov. 13, 1934 2,020,393 Woolrich Nov. l2, 1935 2,038,049 Kirsten Apr. 2l, 1936 2,182,732 Meyer Dec. 5, 1939 2,530,990 Peters Nov. 21, 1950 

